I had painfull forearm spasms last summer when I was doing a bunch of push ups and dips when lifting. But it would seize up after BJJ practice or when I would be driving. As soon as I cut the amount of pushups and dips the spasms subsided. But you should still see a doctor.

If ive taken a break from training for a while sometimes my forearms and neck will hurt a bit. if a doctor checks you out and says nothing is wrong with you, a good remedy i find for muscle pain is after training to alternate showers with really hot and then really cold water on the affected parts.

a good stretch for the forearms is to make fists and bring your hands up level with your head, curling them like a goose's neck so you stretch the back of your hand, and then extend your arms out in front of you as far as you can and repeat a few times.

playing musical instruments with bad technique can definitely lead to over-use injuries, but imo (non medical) it would more likely affect you at the elbow joint. just make sure like in training that you warm up slowly and carefully before you play, and get some proper instruction to make sure you are doing it right. you might want to do your own little warm up before your capoeira training to, just to concentrate a bit more on your trouble areas, and stretch these out gently after training also.

It's always better safe than sorry. If you are worried about it, go to your GP. You don't need to seem to 'man up' if you are worried about an injury. Get it checked out, get OKAY'd with it, and keep training. If there is something wrong, you can get it checked out now, before more damage might occur.

I dunno what health services are like where-ever you live, but in Aus health care is pretty much looked after if you have some Medicare (Private health care), GPs bulk bill so doesn't cost anything to visit them...and I do if things start to bother me...